1. Field
The present embodiments relate to an organic light-emitting diode display device and a driving method thereof.
2. Related Art
Recently, various flat panel display devices have been developed. These flat panel display devices have a reduced weight and bulk and are capable of eliminating disadvantages of a cathode ray tube. Such flat panel display devices include, for example, a liquid crystal display device (hereinafter, referred to as “LCD”), a field emission display device (hereinafter, referred to as “FED”), a plasma display panel (hereinafter, referred to as “PDP”) and an electro-luminescence display device.
In such flat panel display devices, the PDP has a light weight, a small bulk size and a large dimension screen because its structure and manufacturing process are simple. However, the PDP has low light-emission efficiency and large power consumption.
The active matrix LCD employing a thin film transistor (hereinafter, referred to as “TFT”) as a switching device has drawbacks in that it is difficult to increase the dimension screen because a semiconductor process is used. Recently, however, the LCD has an increased demand because it is mainly used for a display device of a notebook personal computer.
The EL device is largely classified into an inorganic EL device and an organic light-emitting diode device depending upon a material of a light-emitting layer, and is a self-luminous device. When compared with the above-mentioned display devices, the EL device has advantages of a fast response speed, large light-emission efficiency, a large brightness and a large viewing angle.
Referring to FIG. 1, the organic light-emitting diode device comprises an anode electrode made from a transparent conductive material on a glass substrate, an organic compound layer disposed on the organic light-emitting diode device, and a cathode electrode made from a conductive metal.
The organic compound layer is comprised of a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL and an electron injection layer.
If a driving voltage is applied to the anode electrode and the cathode electrode, then a hole within the hole injection layer and an electron within the electron injection layer move toward the emission layer, respectively, to excite the emission layer, so that the emission layer emits visible rays. The visible rays generated from the emission layer display a picture or a motion picture.
The organic light-emitting diode device has been applied to a display device of a passive matrix type or to a display of an active matrix type using a TFT as a switching element. The passive matrix type crosses the anode electrode with the cathode electrode to select a light-emitting cell in accordance with a current applied to the electrodes while the active matrix type selectively turns on an active element, for example, a TFT to select a light-emitting cell and maintains a light-emitting of the light-emitting cell using a voltage maintained at a storage capacitor.
FIG. 2 is a circuit diagram equivalently showing one pixel in an organic light-emitting diode display device of an active matrix type.
Referring to FIG. 2, the organic light-emitting diode display device of the active matrix type includes an organic light-emitting diode element OLED, a data line DL and a gate line GL that cross with each other, a switch TFT T2, a driving TFT T1 and a storage capacitor Cst. The driving TFT T1 and the switch TFT T2 are implemented in a p-type MOS-FET.
The switch TFT T2 is turned-on in response to a gate low-level voltage (or a scanning voltage) from the gate line GL to be electrically connected a current path between a source electrode and a drain electrode of the switch TFT T2. The switch TFT T2 maintains an off-state when a voltage on the gate line GL is less than a threshold voltage (hereinafter, referred to as “Vth”) of the switch TFT T2, for example, a gate high-level voltage.
A data voltage from the data line DL is applied, via the source electrode and the drain electrode of the switch TFT T2, a gate electrode and a storage capacitor Cst of the driving TFT T1 during an on-time period of the switch TFT T2. Alternatively, a current path between the source electrode and the drain electrode of the switch TFT T2 is opened during an off-time period of the switch TFT T2 to not apply the data voltage VDL to the driving TFT T1 and the storage capacitor Cst.
The source electrode of the driving TFT T1 is connected to a driving voltage line VL and one end of the storage capacitor Cst. The drain electrode of the driving TFT T1 is connected to the anode electrode of the organic light-emitting diode display OLED. The gate electrode of the driving TFT T1 is connected to the drain electrode of the switch TFT T2. Such a driving TFT T1 adjusts a current amount between the source electrode and the drain electrode in accordance with a gate voltage supplied to the gate electrode, for example, a data voltage to have the organic light-emitting diode display OLED to be emitted at brightness corresponding to the data voltage.
The storage capacitor Cst stores a difference voltage between the data voltage and a high-level electric potential driving voltage VDD, which constantly maintains a voltage applied to the gate electrode of the driving TFT T1 during one frame period.
The organic light-emitting diode display OLED is implemented in the structure as shown in FIG. 1 and includes a cathode electrode connected to the drain electrode of the driving TFT T1 and a cathode electrode supplied with a ground voltage source GND. The organic light-emitting diode display OLED is emitted by a current between a source-drain of the driving TFT T1 defined in accordance with the gate voltage of the driving TFT T1.
The organic light-emitting diode display device as shown in FIG. 2 determines a current flowing into the organic light-emitting diode display OLED in accordance with a characteristics of the driving TFT T1. Accordingly, if the characteristics of the driving TFT T1 are uniform for each pixel, then a picture is displayed with constant brightness characteristics. The characteristics of the driving TFT T1, for example, a threshold voltage characteristic is different at each position in a screen of the manufactured panel. Because a high-level potential driving voltage VDD is dropped by the driving voltage line VL, brightness at each position in the screen even through the same data are supplied to the screen.
FIG. 3 shows a vertical strip phenomenon of a screen generated at the same gray scale data by a voltage drop defined by a threshold voltage deviation of the driving TFT T1 and the driving voltage line VL at the organic light-emitting diode display device of the active matrix type.
For example, as shown in FIG. 4, because a power of laser is instabilized in accordance with in length of time when an amorphous silicon a-Si formed on a TFT substrate of the organic light-emitting diode display device is crystallized in a poly silicon p-Si at a laser crystallization process, the semiconductor characteristics of the TFT substrate are uninform. Because a membranous of a silicon thin film generated at a border between portions irradiated at different time, the scanning and the laser irradiation are performed for the surface of the substrate at a regular interval, the semiconductor characteristics of the TFT substrate are uniform. When the semiconductor characteristics of the TFT substrate generates a deviation depending upon a position, a stripe phenomenon is generated as shown in FIG. 3 and brightness is not uniformly generated at the same gray scale data.